1. Field of the Invention
This invention relates to communication between different networks and, more particularly, to communication between a plurality of local area networks (LANs) including ring-type LANs and, even more particularly, to communication between different networks by detecting a transfer frame circulation in a ring-type LAN.
2. Description of the Related Art
As demand grows for integration of data processing and communication systems, applications for the use of local area networks (LANs) are increasing. Currently, LAN standards are being investigated by the Institute of Electrical and Electronic Engineers (IEEE), the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO), among others, and some standard LANs have already been proposed.
Some known LANs use a standard ring-type topology. A popular ring-type LAN is the token ring or Fiber Distributed Data Interface (FDDI) system controlled by a media access control (MAC) system. In this system, the right of transmission is shifted between nodes by circulating a control frame call token throughout the network. A node desiring to send at least one frame, waits to seize the token. After seizing the token, the node first transmits the frames to be sent and then, to transfer the right of transmission to the next node on the ring, retransmits the token after transmission of the frames. The node later receives the frames after they have circulated around the ring and does not retransmit them, thus eliminating the frames from the ring.
Previously, all information processing devices such as host and terminal units had to be assigned to the same LAN. With an increasing number of information processing devices and applications demanding interaction with one another, accommodation of all information units within only one LAN can be difficult if not impossible. In addition, realizing different application fields and processes on only one LAN is inadequate from the point of view of function and performance. Therefore, more than one LAN is sometimes installed by a LAN user.
A requirement remains for effective transmission of data from a node of one LAN to a node of another LAN. For the control of communication between different types of LANs, a flexible and low cost system for communication between different LANs is needed.
The present invention provides a system for internetwork communications between different types of LANs not heretofore possible. FIG. 1 shows an example of a conventional network structure for mutual connection between two LANs. The network connects LAN I and LAN II through bridge 1, the interface for communication between different networks. Nodes 2, 3 and 4 belong to LAN I and nodes 5, 6 and 7 belong to LAN II in the example of FIG. 1. Bridge 1 is also a node belonging to both LANs I and II and functions as a media access control (MAC) bridge for controlling transmission and reception of internetwork transfer frames, for internetwork mutual communication between, for example, the nodes 3 and 5. Internetwork communication between other LAN nodes is thus possible.
FIG. 2 shows an example of another conventional network structure in which a plurality of networks are mutually connected. Ring-type LAN III is a token ring LAN provided with bridge stations 8 and 9 as well as nodes 10 and 11. LAN III is connected with LANs IV and V through bridge stations 8 and 9. LAN IV is provided with terminals 12, 13, 14 and LAN V is provided with terminals 15, 16, 17. Mutual internetwork communication of internetwork frames over LANs III, IV and V is carried out through bridge stations 8 and 9.
FIG. 3 shows a conventional frame format applied to the Fiber Distributed Data Interface (FDDI) type of LAN. As shown in FIG. 3, each frame is composed of a plurality of fields: a phase synchronization (preamble) PA, a start delimiter SD, a frame control FC, a distant apparatus (destination node) address DA, a self apparatus (start node) address SA, an information part INFO, a frame check sequence FCS, an end delimiter ED, and a frame status FS. The phase synchronization PA is used for phase synchronization upon reception; the start delimiter SD is used for indicating the frame start position; and the end delimiter ED is used for indicating the frame end position.
A node of the known Fiber Distributed Data Interface (FDDI) type of LAN erases and will not retransmit a received frame when the self apparatus (start node) address SA of the received frame matches the address of the node because the node assumes the frame is a frame previously transmitted by itself. Similarly, in Japanese Laid-open patent application (Kokai) No. 61-084940, filed Nov. 11, 1983 and published Jun. 8, 1985 in Japan, after a circulation time has passed since a frame was transmitted from a system node, the received frame is erased without retransmission, because the node assumes the received frame is the frame previously transmitted by itself. However, when bridge 1 or bridges 8 and 9 are shown in FIGS. 1 or 2 are operated like ordinary nodes for intranetwork communication, as above, internetwork communication problems occur.
Problems occur in the internetwork mutual connection network of FIG. 1, for example, when a frame including the self apparatus (start node) address SA, for example, the address of node 3, and the distant apparatus (destination node) address DA, for example, the address of node 5, is formed and transmitted to the LAN I from the node 3, for example, to transfer the frame to node 5 of LAN II from node 3 of LAN I through bridge 1. The frame received by bridge 1, is sent to LAN II causing the desired node 5 to receive it. Bridge 1 then automatically erases the internetwork transfer frame after it has circulated the ring of LAN II. However, when bridge 1 is structured to operate like the ordinary nodes, as described above, problems occur in the event the self apparatus (start node) address SA in the received frame matches the node number or address of bridge 1. The transmitted frame is erroneously erased by bridge 1. Accordingly, the internetwork transfer frame having the self apparatus node address SA=3 cannot be erased from the LAN II and this transfer frame continuously circulates in LAN II. This also occurs when the frame is transferred to nodes 2, 3 or 4 in LAN I from the nodes 5, 6, or 7 in LAN II.
A frame used in a method of formatting a received frame is illustrated in Prior Art FIG. 4. Using the frame illustrated in FIG. 3, when bridge 1 receives a frame transferred to node 5 from node 3, the distant apparatus node address DA in the transfer frame is the address of node 5 and the self apparatus node address SA is the address of node 3. In node 1, the frame is formatted as illustrated in FIG. 4, information INFO.sub.1 is defined as part of information INFO.sub.2 which also includes the original destination DA and start SA addresses; self apparatus node address SA is assigned to the address of bridge 1; and distant apparatus node address DA is assigned to the address of node 5. This new transfer frame is transmitted to the LAN II by bridge 1. In this case, the receiving node 5 (distant apparatus) analyzes the content of information part INFO.sub.2 in the received frame to determine that the self apparatus node address SA is the address of node 3. Each node in a LAN using this type of internetwork communication is required to provide not only the self apparatus address SA, but also the ability to analyze the self apparatus from the content of INFO.sub.2 to deal with two kinds of frame format illustrated in FIGS. 3 and 4. As a result, a more complicated system is required.
When using a counter to monitor the receiving timing of a frame transmitted from the self apparatus in order to erase the frame, complicated procedures are necessary for changing the ring circulation time of all nodes, node by node, due to addition and erasure of a node in the network.